Hess's law: $delta_H$ is path-independent. Three calculation methods: (1) Formation enthalpies: $delta_H$ = sum(n*$delta_{Hf}$ products) - sum(n*$delta_{Hf}$ reactants). Elements in standard state have $delta_{Hf}$ = 0. (2) Bond energies: $delta_H$ = sum(BE bonds broken) - sum(BE bonds formed). Breaking bonds is endothermic (+), forming is exothermic (-). (3) Combustion enthalpies: $delta_H$ = sum(n*$delta_{Hc}$ reactants) - sum(n*$delta_{Hc}$ products). Note the reversal compared to formation! Common error: mixing up the sign convention between formation (products - reactants) and bond energy (reactants - products for bonds).